1. Field of the Invention
The present invention relates to the cleaning method of photoresist stripping, etc. by electrolytic sulfuric acid containing oxidizing agent formed through electrolysis of sulfuric acid applying the conductive diamond anode, and the manufacturing method of semiconductor device.
2. Description of the Related Art
In the so-called wet cleaning technology, where silicon wafer works are objects of cleaning as seen in the semiconductor device manufacturing, persulfuric acid is used as removing agent for used photoresist, metals and organic pollutants. For such persulfuric acid, electrolytic sulfuric acid containing an oxidizing agent formed by electrolysis of sulfuric acid is used.
Conventionally, as the cleaning method for photoresist stripping, etc. applying electrolytic sulfuric acid containing oxidizing agent formed by electrolysis of sulfuric acid, such method is known that persulfuric acid formed through electrolysis of sulfuric acid is mixed with sulfuric acid at a higher concentration and at a higher temperature than said persulfuric acid, for use of photoresist removal from the substrate. (Patent Document 1)
Patent Document 1 describes that if the treatment solution comprising the mixed solution is raised to 120 degree Celsius or more by the dilution heat generated when sulfuric acid of a higher concentration and a higher temperature is diluted with persulfuric acid, the photoresist stripping effect will be enhanced.
Patent Document 1 also describes that since the sulfuric acid used for production of persulfuric acid by electrolysis is low in concentration as 2-11 mol/L. (20-70% by mass), the electrolysis efficiency to form electrolytic sulfuric acid and production efficiency of persulfuric acid are high, but due to low concentration of sulfuric acid contained in the electrolytic sulfuric acid, the stripping efficiency of photoresist is low and it takes a long time for the photoresist stripping, representing the vital defect as a cleaning process.
The inventors of the present invention have invented, and filed for patent, the sulfuric acid electrolysis process to manufacture persulfuric acid by electrolyzing concentrated sulfuric acid at 90% by mass or more of concentration using a conductive diamond anode, as a technology to supply persulfuric acid with a high cleaning effect, continuously and quantitatively at a high efficiency, and the cleaning process for silicon wafer works applying persulfuric acid manufactured by said process. (Patent Document 2) Compared with platinum electrodes widely used so far as electrodes to form persulfate, this conductive diamond electrode, having a larger oxygen generation overpotential, shows a higher efficiency in electrolytic oxidation of sulfuric acid into persulfuric acid, a higher chemical stability and has a longer electrode life.
The process described in Patent Document 2 electrolyzes concentrated sulfuric acid at a concentration at over 90% by mass, and the electrolytic sulfuric acid containing the oxidizing agent formed from the electrolysis reaction of concentrated sulfuric acid, such as peroxomonosulfuric acid, contains less moisture and therefore, is not decomposed through reaction with moisture, capable of stably forming such oxidizing agent as peroxomonosulfuric acid, achieving a high stripping efficiency for photoresist, etc. Moreover, the higher the concentration of sulfuric acid, the higher the stripping effect it achieves in a short time, resulting in a shorter treating time of the particular step in the semiconductor manufacturing process. The electrolytic sulfuric acid formed by the method of Patent Document 2 contains concentrated sulfuric acid at over 90% by mass and therefore the effect is high.
Electrolytic sulfuric acid means the electrolysis product obtained through electrolysis of sulfuric acid, comprising sulfuric acid added as electrolyte, and substances containing oxidizing agent formed through electrolysis, such as peroxomonosulfuric acid, peroxodisulfuric acid, ozone, and hydrogen peroxide. Incidentally, peroxomonosulfuric acid and peroxodisulfuric acid are collectively called persulfuric acid.
It is known that when such oxidizing agents as peroxomonosulfuric acid, peroxodisulfuric acid, ozone, and hydrogen peroxide are prepared through electrolysis of sulfuric acid, a lower concentration of sulfuric acid supplied to the sulfuric acid electrolytic cell will form oxidizing agents at a higher efficiency, resulting in a higher electrolytic current efficiency. Therefore, as in the method disclosed in Patent Document 2, if a large amount of oxidizing agent is intended to obtain through direct electrolysis of concentrated sulfuric acid as with the case of applying sulfuric acid at a low concentration, additional measures are required, such as prolonged electrolysis time, increased current density, or increased number of sulfuric acid electrolytic cell. This method, however, leads to various problems including poor productivity, scaling up of equipment, increased loading to sulfuric acid electrolytic cells, increased power consumption, and shortened life of hardware.
To improve the stripping property in the cleaning process, sulfuric acid to be supplied to the sulfuric acid electrolytic cell should, preferably, be concentrated sulfuric acid at a high concentration; while, on the other hand, to obtain oxidizing agent effectively by improving electrolytic property, sulfuric acid should, preferably, be at a low concentration. Thus, opposing properties are required for co-existence of the stripping property and the electrolytic property.                [Patent Document 1] Tokkai 2008-66464 Patent Gazette        [Patent Document 2] Tokkai 2008-19507 Patent Gazette        